There are certain non-linear impedance characteristics that are an inherent property of man-made objects that contain metal to metal, semiconductor to semiconductor, and metal to semiconductor interfaces. When such man-made objects are illuminated by an electromagnetic signal, the rectification properties of the nonlinear impedances cause new signals to be generated at frequencies that are exact multiples of the frequency of the original signal. These new signals are radiated, and can be detected with a superheterodyne receiver like that used in conventional radars.
A limited number of tactical radar units were developed to exploit these phenomena. One of the earliest is the Metal Target Re-Radiation (METRRA) system developed for the U.S. Army. In this system, three experimental helicopter and vehicle-mounted radars were developed to detect stationary military targets (tanks, vehicles, rifles, and weapon caches) hidden by foliage, a difficult target environment for conventional radar. They successfully demonstrated a one kilometer range capability by transmitting a 400 MHz signal (nominally) and receiving a 1200 MHz signal in return, i.e., the third harmonic.
More recently, a system was developed that incorporated microprocessor technology for signal identification and discrimination. Swept frequency techniques and a directive antenna system were used for determining range. This system was, however, very expensive and had problems associated with nonlinear impedance effects in the electrical connections that linked the transmit circuitry to the antenna array. Without extremely careful, labor-intensive assembly procedures, the system would respond to the harmonics generated within the antenna interface assembly of the device itself and overwhelm or disguise signal responses from legitimate targets.
An even more recent development is a high-power harmonic radar intended for airborne use with large (kilometers) standoff distance. One obvious application for such a system would be the detection of man-made facilities (e.g., drug labs) in supposedly wild jungle areas. However, the success of such a system would be heavily dependent on ambient clutter. In Vietnam, for example, the metal shroud lines used on parachute flares made it possible to detect nonlinear junctions nearly everywhere that U.S. troops had been.
With respect to the problems addressed by the present invention, however, standoff distances are moderate, and power levels need to be comparable to those used in cell phones to stay within safe human exposure limits. None of the aforementioned systems are portable, low power systems that have robust target discrimination capabilities for reliably characterizing manmade objects possessing nonlinear junctions.
What is needed is a radar system that can rapidly detect and reliably identify targets such as concealed weapons and electronics. The targets can be carried by persons either walking through a fixed portal/doorway or walking or congregating in a foyer, entranceway, or other open area. The system needs to be able to achieve a high probability of detection with a low false alarm rate and automatically discriminate between weapons, electronic assemblies and clutter.
The system proposed for detecting concealed weapons, electronics, and other man-made objects is a harmonic radar nonlinear junction detector. The system utilizes state-of-the art wireless technology, circuit fabrication, signal synthesis, and computer processing techniques to detect and characterize man-made objects possessing nonlinear junctions.
For convenience, the present invention is referred to as a Concealed Weapon and Electronics Radar (CWER) system. CWER is an advanced harmonic radar transmitting a pair of low power (safe for human exposure) waveforms. A receiver within the CWER system is coherently tuned to harmonics of the transmitted frequencies of the waveforms to detect manmade metal objects and electronics that contain non-linear junctions. The CWER receiver is also capable of receiving inter-modulation products reflected from the man-made objects that are a result of using two incident signals.
In its most basic operation, a harmonic radar will transmit a single waveform at a nominal carrier frequency of 1 GHz and receive 2nd harmonic (2 GHz) returns from electronic devices and 3rd harmonic (3 GHz) returns from metal devices. A single frequency system has adequate detection capability with the ability to distinguish between electronic and metal objects. However, its ability to discriminate targets from clutter objects on the person (e.g., keys, coins, zippers, buckles, etc.) or clutter objects in or near by a doorway (e.g., hinges, springs, and doorknobs) is limited. Further, a single frequency system has a very limited capability with respect to classifying different types of targets such as guns and knives.
To address the inherent deficiencies of a single frequency system, the present invention uses two signal sources generating user-definable waveforms of variable frequencies in order to provide enhanced discrimination and target identification abilities via the processing of returned inter-modulation products.